Terms and Definitions
A-weighted sound pressure level – dB(A)
The A-weighted sound pressure level is the weighted average value of the sound pressure level (dB) as a function of the frequency of a sound. The weighting takes into account the ability of the human auditory system to perceive sound pressure levels or tones of different frequencies to a different degree. This sensitivity is particularly pronounced in the medium frequency range, i.e. the range of human speech. Nearly all regulations and guidelines indicate values expressed in dB(A).
The acoustic quality of a room refers to its suitability for a particular use. It is influenced by the properties of the boundary surfaces (walls, ceiling, floor) and the furnishings and by persons present in the room.
Auralisation is a method for simulating the acoustic properties of a room. With this method, the effects of certain acoustic treatments can be "auralised" as early as the design stage.
Background noise level
Usually, sounds which do not contain any meaningful information are referred to as background noise (e.g. noise from air conditioning or traffic). The background noise level is measured in dB or, by weighting its frequencies in accordance with the human auditory system, in dB(A).
The background noise level indicates the sound pressure level which has been exceeded during 95% of the measurement period. It has a direct effect on speech intelligibility.
Building acoustics is a branch of building physics, or acoustics, which deals with the effect of the structural conditions on the propagation of sound between the rooms of a building or between the interior of a room and the outside of the building.
When a sound wave encounters an obstacle its direction changes at the boundaries of this obstacle. Part of the sound energy thus enters the shadow region behind the obstacle. This portion of the sound, which is basically "deflected" at the edge, is referred to as diffracted sound.
Logarithmically defined unit of measurement which expresses the sound pressure level. The relevant scale for human beings is 0 dB to 140 dB. 0 dB refers to a sound pressure of 20µPa.
Equivalent sound absorption area
The equivalent sound absorption area A is defined as the product of the sound absorption coefficient α of a material and the surface S of this material.
A flutter echo occurs when a sound signal moves back and forth several times between at least two heavily reflecting surfaces. It can be perceived subjectively: a shot or the clapping of hands sound like machine gun fire which becomes weaker and weaker. Flutter echoes are usually perceived as annoying and should be avoided. This can be achieved by the geometrical design of the room and/or by covering part of the reflecting surfaces with absorbing material.
Frequency indicates the number of sound pressure changes per second. Sound events with a high frequency are perceived by the human ear as high-pitched tones, sound events with a low frequency as low-pitched tones. Sounds such as noise, road traffic, etc., normally comprise a great number of frequencies. The measurement unit of frequency is hertz (Hz), 1 Hz = 1/s. Human speech is in the range between 250 Hz and 2000 Hz. The audible range of human beings is between 20 Hz and 20000 Hz.
Noise comprises all sounds which, due to their loudness and structure, are considered as harmful or annoying or stressful for human beings and the environment. It depends on the condition, preferences and mood of a person whether sounds are perceived as noise or not. The perception of sounds as noise and the way in which people are affected by it depend, on the one hand, on physically measurable quantities such as the sound pressure level, pitch of a tone, tonality and impulsiveness. On the other hand, certain subjective factors also play a role: at bedtime noise is perceived as extremely annoying. The same is true for activities which require a high level of concentration. If we like certain sounds, we will not perceive them as annoying even at high volumes; sounds which we do not like are annoying to us even at low volumes (e.g. certain types of music). Furthermore, how we feel at a particular time also influences our sensitivity to noise. If an activity is disrupted or disturbed by one or more sounds, this is referred to as noise pollution. We are particularly sensitive to noise if verbal communication is affected, e.g. if a loud conversation at the neighbouring table makes it difficult for us to listen, and if we have to concentrate or want to sleep.
Acoustic parameters such as the sound pressure level or the sound absorption coefficient are usually expressed in increments of octave band and one-third octave band. The precise knowledge of acoustic properties in the smallest possible frequency steps of sound is a prerequisite for a detailed acoustic design.
For room acoustics the relevant octave band centre frequencies are 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz. The octave bands are obtained by doubling the previous frequency. Each octave comprises three one-third octave values (see also single number ratings).
Omni-directional sound sources
Spherical sound sources are basically those sound sources which radiate uniformly into all three directions of space. Since hardly any loudspeaker exhibits an omni-directional characteristic in its near field, for certain applications in acoustic measurement technology there are special measurement loudspeakers, so-called dodecahedron loudspeakers, which incorporate twelve individual loudspeakers in a nearly spherical arrangement, thus forming an approximately omni-directional sound source between 100 Hz and 4000 Hz.
Porous absorbers comprise, for example, mineral fibres, foams, carpets, fabrics, etc. The effect of the porous absorbers is due to the fact that sound is able to enter the open structures of the material where, by the friction of air particles, the sound energy is converted into thermal energy at the surface of the pores. Porous absorbers achieve their best effect at medium and high frequencies.
Branch of acoustics or noise effect research which deals with the subjective perception of objectively present sound signals. Furthermore, psychoacoustics studies the influence of a listener's personal attitudes and expectations on the perception of sound events.
Rating level (Lr)
The rating level Lr (L for "level", r for "rating") is the relevant parameter for objectively assessing the noise impact at a workplace. Apart from weighting the sound pressure level as a function of the frequency (see A-weighted sound pressure level), a determination of the sound pressure level takes into account certain adjustments which depend on the characteristic of the sound (e.g. impulsiveness or clear prominence of individual tones) and its duration of impact. The rating level is also expressed in dB(A).
This term comprises all types of absorbers using a resonance mechanism such as an enclosed air volume or a vibrating surface. Resonant absorbers are mainly suitable for absorbing sound of medium to low frequencies. The maximum effect of resonance absorbers is usually restricted to a certain frequency range (see also "porous absorbers").
Reverberation chamber are special laboratory rooms with walls which reflect the incident sound waves to a very high degree. Reverberation chamber have particularly long reverberation times across the entire frequency range.
Reverberation chamber method
The reverberation chamber method is used for determining the frequency-dependent sound absorption coefficient. A sample of the material to be tested is placed into the reverberation chamber. The sound absorption of a material can then be calculated from the change in the reverberation time of the room.
Put simply, the reverberation time indicates the period of time it takes for a sound event to become inaudible. Technically, the reverberation time T has been defined as the time required for the sound pressure level in space to decay by 60 dB.
Room acoustics is a branch of acoustics which deals with the effect of the structural conditions of a room on the sound events occurring in this room. Rooms in terms of this definition may be concert halls, theatres, class rooms, studios, churches, but also office rooms, call centres or conference rooms, in which acoustic presentations (speech and music) or communication generally occur. The central issue of room acoustics is to determine which surfaces can be used to create optimum listening conditions. In this context, the most important property of the materials is their sound absorption.
If the volume and the total equivalent sound absorption area of a room are known, the reverberation time can be estimated using the Sabine formula, where "T" is the reverberation time, "V" is the volume of the room and "A" is the total equivalent sound absorption area.
The close relationship between the volume of a room, the sound absorption of the surfaces of this room, and the reverberation time was discovered the physicist Wallace Clement Sabine (1868 - 1919). He found out that the reverberation time T is proportional to the room volume V and inversely proportional to the equivalent sound absorption area A:
T = 0,163 x V / A (in metric units).
The equivalent sound absorption area A is the sum of all surfaces S present in the room, each multiplied by its corresponding sound absorption coefficient α:
A = α1S1 + α2S2 + α3S3 + … + αnSn
Single number ratings of sound absorption
So-called single number ratings are used for a simplified representation of the frequency-dependent parameter of the sound absorption coefficient as well as for a rough comparison of different sound absorbers. In Europe, the "weighted sound absorption coefficient" αw in accordance with the European / International standard EN ISO 11654 is commonly used. In the US, the NRC and SAA values are widely used. All of the above values are based on measurements of the sound absorption in one-third octave and octave bands. For a detailed acoustic design of a room it is necessary to know these sound absorption values precisely in one-third octave or at least in octave bands (see "octaves").
Sound absorbers are materials which attenuate incident sound or convert it into other forms of energy. A distinction has to be made between porous absorbers and resonant absorbers or combinations of these absorber types.
Sound absorption coefficient α
The sound absorption coefficient a of a material indicates the amount of the absorbed portion of the total incident sound. α = 0 means that no absorption occurs; the entire incident sound is reflected. If α = 0,5, 50 % of the sound energy is absorbed and 50 % is reflected. If α = 1, the entire incident sound is absorbed, there is no longer any reflection.
Sound attenuation describes the ability of materials to absorb sound or to convert the sound energy present into other forms of energy, i.e. ultimately into thermal energy (see also "sound insulation").
A sound barrier is basically an obstacle which interrupts the direct propagation of sound from a source to a receiver. It can consist in a movable partition or an attachment to be placed on top of a desk. Cabinets and other large-surface pieces of furniture can also function as sound partitions. Sound partitions can be provided with a sound absorbing surface which additionally reduces the propagation of sound.
General term for tones, music, bangs, noise, crackling, etc.
Sound masking specifically uses natural (e.g. birds' twittering) or artificial (e.g. noise) sounds in order to blanket other sounds. This method can be used, for example, to drown out information-containing sounds if the other background noise is too weak to mask them.
All sound events have in common the fact that they cause slight variations in air pressure which can propagate in elastic media such as air or water. We therefore refer to the sound pressure of a tone. The heavier the pressure variations are, the louder is the sound event. The faster the variations occur, the higher is the frequency.
Sound pressure level (Lp)
The sound pressure level (L for level and p for pressure) is a logarithmic quantity for describing the intensity of a sound event. The sound pressure level is often also referred to as "sound level", which is actually not quite correct. The sound pressure level is expressed in decibels (abbreviated as dB). Sound pressures are measured using microphones. The measurable level range starts at just below 0 dB and ends at approximately 150 to 160 dB.
The sound spectrum describes the frequency composition of the sound. Pure tones are sound events of a single frequency. A superposition of tones of different frequencies is referred to as noise or sound.
Sound transmission loss
Sound insulation or sound transmission loss refers to the restriction of the propagation of sound through the boundaries of a room. Sound insulation is, therefore, a measure to separate rooms acoustically from unwanted sound from adjacent rooms or the outside. This has nothing to do, however, with the required acoustic sound attenuation within a room (see also "sound absorption"). Sound transmission loss is a fundamental parameter of building acoustics.
A distinction has to be made between airborne sound and impact sound. Airborne sound is created by sound sources present in the room which are not immediately connected to the boundary surfaces, e.g. people who are talking. Impact sound, on the other hand, results from structure-borne sound (footfalls, knocking), which in turn excites the walls or ceilings to radiate airborne sound. Airborne sound insulation and impact sound insulation both have to fulfil the requirements established in relevant building laws.
Variations in air pressure which are caused by sound events are referred to as sound waves. The length of the sound waves defines the frequency and their height defines the level. Long sound waves have a low frequency and are perceived as low-pitched tones. Short sound waves have a high frequency and are perceived as high-pitched tones.
In air, a 100 Hz wave has an extension of 3.40 metres, whereas a 5000 Hz wave has an extension of approximately 7 centimetres.